This invention relates generally to electronic photodiode devices, and more particularly relates to fuse protection in an array of photodiodes.
Arrays of electronic photodiodes, and in particular avalanche photodiode (APD) arrays, have found extensive use in a variety of photon-counting, imaging, and communications applications. When operating in so-called Geiger-mode, APDs are biased above the diode breakdown voltage; under this condition, the generation of an electron-hole pair, either through the absorption of light or through thermal generation, can cause the diode to breakdown, producing a rapid rise in electrical current. This rise in current is large enough to directly drive CMOS digital logic without the need for external signal amplification. As a result, the APD array provides highly sensitive radiation detection with microelectronic circuitry.
One challenge to successful and robust integration of a Geiger-mode APD array with an electronic readout integrated circuit (ROIC) for processing electrical signals from the array is the possibility of electrical short circuiting of one or more photodiodes in an APD array during APD operation. If an electrical short-circuited photodiode diode in the array is electrically connected to a ROIC when the short circuit occurs, the electrical short circuiting can diminish the readout circuit performance or even catastrophically destroy the APD-ROIC assembly. In addition, an APD array as-fabricated can include one or more faulty photodiodes that are inadvertently electrically short-circuited due to a microfabrication error. In an APD array with a relatively small population of photodiodes, a small number of shorted photodiodes results in a slightly lower yield of operational APD array assemblies. But for relatively larger APD arrays including a large population of photodiodes, the likelihood of a fabrication-produced faulty photodiode in the array is relatively high and can be quite costly.
In order to minimize the likelihood that an electrically short-circuited or degraded photodiode in an APD array is electrically connected to the APD array ROIC, each photodiode in the array can be both visually and electrically prescreened after fabrication, prior to packaging, to check for defects. Following this prescreening, the electrical connection to defective photodiodes can be spot-knocked to disable electrical contact between each defective photodiode and the ROIC. While this technique is fairly effective at preventing the connection of electrically short-circuited photodiodes in APD-ROIC assemblies, there are several drawbacks associated with the device prescreening and spot-knocking process. The visual and electrical inspection of each photodiode is time consuming and limits array fabrication throughput. In addition, electrical probing of APD photodiode devices can, in and of itself, introduce damage to the APD array that is undetected prior to operation.
More importantly, however, even if careful and successful APD photodiode post-fabrication screening can be achieved, the electrical short-circuiting of photodiodes can occur during APD operation, as explained above. APD array exposure to radiation, unexpected operational voltage surges, material fatigue, and other factors can cause such short circuiting of photodiodes. Indeed any exposure of an APD array to environmental or operational conditions that produce voltages exceeding the photodiode breakdown voltage can limit photodiode performance and can damage or destroy the APD array. Without individual photodiode short circuit protection, the APD array can be rendered inoperable or catastrophically damaged.